This invention relates to a method and an apparatus for processing a sheet of a thermographic material, in particular an imaged sheet of a photothermographic material. Applications comprise medical fields (e.g., diagnosis) as well as graphical fields (e.g., four-color printing).
Thermally developable silver-containing materials for making images by means of exposure and then heating are referred to as photothermographic materials and are generally known (e.g., xe2x80x9cDry Silver(copyright)xe2x80x9d materials from Minnesota Mining and Manufacturing Company). A typical composition of such thermographically image-forming elements contains photosensitive silver halides combined with an oxidation-reduction combination of, for example, an organic silver salt and a reducing agent therefore. These combinations are described, for example, in U.S. Pat. No. 3,457,075 (Morgan) and in xe2x80x9cHandbook of Imaging Sciencexe2x80x9d by D. A. Morgan, ed. A. R. Diamond, published by Marcel Dekker, 1991, page 43.
A review of thermographic systems is given in the book entitled xe2x80x9cImaging systemsxe2x80x9d by Kurt I. Jacobson and Ralph E. Jacobson, The Focal Press, London and New York, 1976, in Chapter V under the title xe2x80x9cSystems based on unconventional processingxe2x80x9d and in Chapter VII under the title xe2x80x9cPhotothermographyxe2x80x9d.
Photothermographic image-forming elements are typically imaged by an imagewise exposure, for example, in contact with an original or after electronic image processing with the aid of a laser, as a result of which a latent image is formed on the silver halide. Further information about such imagewise exposures can be found in EP 810 467 A (to Agfa-Gevaert N.V.).
In a heating step which then follows, the latent image formed exerts a catalytic influence on the oxidation-reduction reaction between the reducing agent and the nonphotosensitive organic silver salt, usually silver behenate, as a result of which a visible density is formed at the exposed points. Further information about the thermographic materials can be found, for example, in the above mentioned patent EP 810 467 A.
The development of photothermographic image-forming elements often poses practical problems. A first problem is that heat development causes a plastic film support to deform irregularly, losing flatness.
A second problem is that heat development often degrades dimensional stability. As the developing temperature rises, plastic film used as the support undergoes thermal shrinkage or expansion, incurring dimensional changes. Dimensional changes can result in wrinkling. Moreover, such dimensional changes are especially undesirable in preparing printing plates, because color shift and noise associated with white or black lines may appear in the printed matter.
In the prior art, many solutions for this dimensional problem have been disclosed, comprising the use as a support of a material which experiences a minimal dimensional change at elevated temperatures. All of these materials have their disadvantages (e.g., solvent crazing, low transparency in ultra-violet (UV), high cost, etc.)
For example, EP 0 803 765 (to Fuji Photo Film) discloses a specially prepared type of polycarbonate, having high transparency and light transmission in the UV region, recommended as a printing plate film support, and EP 0 803 766 (to Fuji Photo Film) discloses a photothermographic material comprising a support in the form of a plastic film having a glass transition temperature of at least 90xc2x0 C.
JP 08211 547 (to 3M) describes a special type of thermographic material is disclosed which is made dimensionally stable by a specific heat treatment of the polymer support.
Among the polyesters, poly-ethylene-terephthalate (PET) is a widely used and inexpensive material. However, it is not dimensionally stable at elevated temperatures. Dimensional stability of PET can be improved by a thermal stabilization, thus rendering a thermally stabilized poly-ethylene-terephthalate film.
In xe2x80x9cPlastics Materialsxe2x80x9d, 4th edition by J. A. Brydson, Butterworth Scientific, 1982, pp. 649-650, thermal stabilization of a poly-ethylene-terephthalate film PET is described. Also C. J. Heffelfinger and K. L. Knox, in xe2x80x9cThe Science and Technology of Polymer Filmsxe2x80x9d Volume II, edited by Orville J. Sweeting, Wiley-Interscience, New York, 1971, pp. 616-618, describes thermal stabilization of PET by heat setting.
U.S. Pat. No. 2,779,684 (to Du Pont de Nemours) discloses a polyester film with improved dimensional stability that does not show any significant shrinkage when exposed to a temperature of 120xc2x0 C. for five minutes under conditions of no tension.
As one can see from the above, many solutions to the problem of dimensional stability have been disclosed which relate to the photothermographic material itself or to its support, or to a special method of preparation. However, in practice, such heat setting produces sheets which still deform too much during thermal processing of an imaged sheet.
Belt- and drum-processors, as disclosed, i.e., in U.S. Pat. No. 5,975,772 (to Fuji Photo Film), may provide a good temperature homogeneity, but they do not allow to process a thermographic material reaching a dimensional stability that is sufficient for e.g., 4-color-printing.
In WO 97/28488 and in WO 97/28489 (both to 3M), a thermal processor is disclosed which comprises an oven and a cooling chamber, more particularly a two-zone configured oven and a two-section configured cooling chamber.
This two-zone configuration results in uneven physical and thermal contact. Indeed, in the second zone of this oven, processing heat is transmitted to the upper side of the photothermographic material by convection, whereas processing heat is transmitted to the lower side of the photothermographic material both by conduction and by convection, which results in a degree of thermal asymmetry in the heating of the two sides of the photothermographic material. By consequence, for some highly sensitive kind of photothermographic materials the imaging quality may decrease, e.g., density unevenness may appear.
Moreover, film transport by means of rollers as disclosed e.g., in WO 97/28488 and in WO 97/28489 has further disadvantages: (i) due to a thermal discharge or unload of the roller, a repetition mark (comprising a mark per revolution of a roller) or a troublesome pattern is perceptible on the photothermographic material, (ii) in case of dust particles or flaws being present on a roller, repetitive pinholes appear on the thermographic material, (iii) automatic-cleaning of the apparatus-rollers is rather difficult to achieve; and (iv) jams of photothermographic material occur more frequently and are less easy to solve.
In summary, the prior art still needs a solution to the problem of dimensional stability of the photothermographic material while thermally processing.
The present application presents an alternate thermally processing with good dimensional stability and without undesirable density differences.
In particular, the present invention does not need a complicated photothermographic material, nor a special method of preparation for the photothermographic material.
The object of this invention is to provide a method for thermally processing a thermographic material with improved dimensional stability. Other objects and advantages of the present invention will become clear from the detailed description, drawings, examples and experiments.
We have discovered that these objectives can be achieved by using a method for thermally processing a sheet of a thermographic material m, comprising the steps of supplying a sheet of a thermographic material having an imaging element le to a thermal processor having a processing chamber, heating the processing chamber to a predetermined processing temperature Tp, transporting the sheet through the processing chamber, exporting the sheet out of the thermal processor such in that the transporting of the sheet through the processing chamber is carried out in a sinuous way by transporting means comprising a first belt and a second belt, wherein during the transporting of the sheet through the processing chamber, the first belt is in contact with a first side of the sheet and the second belt is in contact with a second side of the sheet, opposite to the first side.